Troubletwisters 1 - Garth Nix

Twin siblings Jack and Jaide discover they are pivotal to a secret supernatural organisation that protects the earth from marauding Evil! Portland might seem like a quiet coastal town, and their grandmother is perhaps no dottier than anyone else's, but it soon becomes apparent that the strange things going on around them are anything BUT ordinary. It's all very well discovering that you suddenly have magical powers, but when you don't know exactly what they are, or how to use them, then facing impending peril doesn't seem like a very good idea at all...